The fluidity can be defined as that which pertains to the rate and extent of phospholipid acyl chain excursion away from initial chain orientation during the lifetime of the fluorescence excited state, usually several nanoseconds. The proper fluidity is expected to influence membrane permeability properties and to be required for normal or biological activity of cell membrane. In this experiment, we will explore the effects of the quat on several aspects of bilayer physical state by using fluorescence spectroscopy of the DPH-containing membranes. It will offer a deeper insight into the origin of the fluorescence signal changes accompanying membrane damage due to the quat and help identify which physical properties of the membrane are modified by this compound.